Motor vehicle transmissions of the type addressed by this invention include several fluid operated torque transmitting devices, referred to herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Nt/No, where Nt is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios.
Shifting between the various speed ratios generally involves disengaging a clutch associated with the current or actual speed ratio Ract, and engaging a clutch associated with the desired speed ratio Rdes. The clutch to be released is referred to as the off-going clutch, while the clutch to be engaged is referred to as the on-coming clutch. Shifts of this type are referred to as clutch-to-clutch in that no speed responsive or freewheeling elements are used.
Shifting is initiated in response to a comparison between measured and predetermined values of one or more load condition parameters. The parameters typically include vehicle speed so that the transmission is successively upshifted to the upper speed ratios as the vehicle speed is increased and successively downshifted to the lower speed ratios as the vehicle speed is decreased.
The present invention is directed to the control of clutch-to-clutch downshifting during coast conditions--that is, where the vehicle speed is decreasing, with or without application of the service brakes, at closed or light engine throttle settings. When performing coast downshifts, one objective is to time the shift so as to minimize driveline disruption. This means that the speed differential across the on-coming clutch should be at or near zero at the time of clutch engagement. Another objective is to maintain the establishment of a speed ratio which will provide adequate performance in the event the operator terminates the coast condition by increasing the engine throttle setting. This means that successive shifting should occur as the vehicle slows to a stop and that the neutral intervals between disengagement of an off-going clutch and engagement of an on-coming clutch should be minimized.
The above-noted coast-sync-coast shift controls were addressed by the clutch-to-clutch shift control method disclosed and claimed in the U.S. Pat. No. 5,036,729 to Nitz et al., assigned to General Motors Corporation, and issued on Aug. 6, 1991. According to that control method, the coast downshifts are initiated at a point where, without the employed engine control contained therein for synchronizing the engine speed with the transmission input speed, the input speed would otherwise exceed the engine speed. In automotive terms, the shift is referred to as a coast-sync-coast shift since (1) it is initiated at a coast condition where the input speed is higher than the engine speed, (2) it is completed at a synchronous condition where the speed differential across the on-coming clutch is at or near zero, and (3) a coast condition recurs immediately after the shift.
In operation, the coast-sync-coast downshift comprises Preamble, Neutral, Fill and Completion phases. In the Preamble phase, the torque converter is released (if locked), and the engine control is adjusted to progressively increase the engine speed. Engine control may be achieved via adjustment of an idle air control (IAC) unit which admits a controlled amount of air downstream of the engine throttle for idle speed regulation. The IAC unit is adjusted to a limit value which defines a known flow condition, the limit value being scheduled in relation to a measure of the barometric pressure so that the known flow condition occurs regardless of altitude. When the speed differential across the torque converter indicates that the engine torque is sufficient to accelerate the input shaft of the transmission, the control enters the Neutral phase.
In the Neutral phase, the off-going clutch is disengaged to allow the transmission input speed to rise toward the synchronous speed of the lower speed ratio. When the input speed nears the synchronous speed, the engine control is removed to avoid overshooting and the control enters the Fill phase.
In the Fill phase, the on-coming clutch is filled with transmission fluid in preparation for engagement, and the engine control is modulated, if necessary, to maintain the input speed substantially at the synchronous speed. When the on-coming clutch is ready for engagement, the control enters the Completion phase during which the engine control is returned to its normal setting and the pressure supplied to the on-coming clutch is progressively increased to engage the clutch and complete the shift.
A difficulty arises with the above-described transmission controls during a downshift when engine speed flares beyond the synchronous speed. In this situation, the scheduled pressure may be inadequate to control engine torque. This is particularly significant if the speed flare occurs over an extended period of time as the inadequacy of torque containment by the on-coming clutch pressure can be detrimental to the on-coming clutch. One manner of addressing the above situation is to apply the on-coming clutch at high pressure in order to bring the flare immediately under control. This, however, may result in an unpleasant shift bump readily perceived by the operator. An alternative manner of addressing the situation and further substantially limiting or eliminating any perceived bump attempts to control the on-coming clutch torque capacity to smoothly take-up the flare. Proper pressure scheduling may be difficult to control due to the dynamic nature of any given flare event and consequently even small errors toward under torque capacity will lead to continued flare and possible clutch damage if the flare persists for an excessive period. Additionally, with either of the two techniques described above, the flare which occurs may produce objectional noise related to the severity thereof.